NIRSpec pipeline concept Guido De Marchi, Tracy Beck, Torsten Böker Instrument characteristics multi-object spectrograph --> every detector pixel sees every wavelength reflective optics (incl. dispersive elements) --> large, variable slit curvature used on a diffraction-limited telescope --> PSF varies with l wide wavelength range (0.6 - 5 mm) --> chromatic slit losses off-axis telescope and wide field of view --> significant distortion

NIRSpec pipeline concept Active MSA area Mounting frame Fixed slits and IFU aperture Detector array Direction of dispersion

NIRSpec pipeline concept Guido De Marchi, Tracy Beck, Torsten Böker Instrument characteristics multi-object spectrograph --> every detector pixel sees every wavelength reflective optics (incl. dispersive elements) --> large, variable slit curvature used on a diffraction-limited telescope --> PSF varies with l wide wavelength range (0.6 - 5 mm) --> chromatic slit losses off-axis telescope and wide field of view --> significant distortion Headaches.....

A first outline of the NIRSpec pipeline RAW DATA BIAS & DARK SUBTRACTION PIXEL-TO-PIXEL DQE CORR. LOCATE EXTR. WINDOW THROUGHPUT CORRECTION (incl. L-flat, blaze function, transmission of optics, & “default” chromatic slit loss) MSA CONFIG. & DISTORTION MAP GW TELEMETRY GEOMETRIC DISTORTION (spatial) FINAL l CALIBRATION (dispersion solution) “DELTA” CHROMATIC SLIT LOSS CORR. ABSOLUTE FLUX CALIBRATION EXTRACT 1-D SPECTRUM SUBTRACT BACKGROUND FINAL 1D SPECTRUM FLATFIELD REF. CUBE GRATING EQUATION PHOTFLAM KEYWORD BIAS/DARK REF. FRAMES “P-FLAT” REF. FRAME LINEARITY CORR. LINEARITY (Assuming no l-dependence) One window for every open shutter….

Defining the extraction windows MSA mask Red: object, green: background Each spectrum has “extraction box” on FPA Extraction boxes overlap, possible “spill-over”

An outline of the NIRSpec pipeline RAW DATA BIAS & DARK SUBTRACTION PIXEL-TO-PIXEL DQE CORR. LOCATE EXTR. WINDOW THROUGHPUT CORRECTION (incl. L-flat, blaze function, transmission of optics, & “default” chromatic slit loss) MSA CONFIG. & DISTORTION MAP GW TELEMETRY GEOMETRIC DISTORTION (spatial) FINAL l CALIBRATION (dispersion solution) “DELTA” CHROMATIC SLIT LOSS CORR. ABSOLUTE FLUX CALIBRATION EXTRACT 1-D SPECTRUM SUBTRACT BACKGROUND FINAL 1D SPECTRUM FLATFIELD REF. CUBE GRATING EQUATION PHOTFLAM KEYWORD BIAS/DARK REF. FRAMES “P-FLAT” REF. FRAME LINEARITY CORR. LINEARITY (Assuming no l-dependence) One window for every open shutter….

Operations on the extraction windows Same as those carried out for traditional ground-based MOS flat-field (uniformity of detector response): f(l), f(x,y) tracing the spectrum and rectifying it: f(l), f(x,y) wavelength calibration: f(l), f(x,y) flux calibration (throughput of optics and gratings): f(l), f(x,y) absolute flux calibration But wait… Because of NIRSpec’s design (e.g. MSA always in the way), some of the steps above must be carried out simultaneously, require calibration measurements that are intertwined

Throughput correction of “Flat-fielding” NIRSpec spectra Throughput correction of ^ Need a “throughput” data cube (for each filter/grating combination) Output (assuming a source with flat spectrum) l ---> x ---> y ---> l ---> l ---> Goal: to correct for the total instrumental throughput variations, both as a function of wavelength (e.g. optics transmission, blaze function) and field angle (e.g. DQE, vignetting).

Contributions to the “Throughput correction” reflectivity of all mirrors: f(l), f(x,y) transmission curves of filters: f(l) blaze function of gratings: f(l), f(x,y) large-scale response of detector (L-flat): f(l), f(x,y) All contributions are measured at component level, and built into a physical/optical instrument model. Once NIRSpec is assembled, they cannot be measured individually. However, once a shutter has been specified, all of them are - in principle - deterministic, and can be accurately modeled. Using the instrument model, all these effects will be corrected simultaneously. But wait……

The bummer: chromatic slit loss Fixed slit size, but variable PSF width……. 1 mm 3 mm 5 mm …… causes “flaring” and intensity gradient: A “default” correction for e.g. a perfectly centered point source can be included in throughput correction. The user needs to optimize this correction later….. l --->

An outline of the NIRSpec pipeline RAW DATA BIAS & DARK SUBTRACTION PIXEL-TO-PIXEL DQE CORR. LOCATE EXTR. WINDOW THROUGHPUT CORRECTION (incl. L-flat, blaze function, transmission of optics, & “default” chromatic slit loss) MSA CONFIG. & DISTORTION MAP GW TELEMETRY GEOMETRIC DISTORTION (spatial) FINAL l CALIBRATION (dispersion solution) “DELTA” CHROMATIC SLIT LOSS CORR. ABSOLUTE FLUX CALIBRATION EXTRACT 1-D SPECTRUM SUBTRACT BACKGROUND FINAL 1D SPECTRUM FLATFIELD REF. CUBE GRATING EQUATION PHOTFLAM KEYWORD BIAS/DARK REF. FRAMES “P-FLAT” REF. FRAME LINEARITY CORR. LINEARITY (Assuming no l-dependence) One window for every open shutter…. (output is re-sampled grid)

Rectifying NIRSpec Spectra Slit is curved (function of field angle) Lines of constant l spread over multiple pixel columns l1 l2 li Use “Drizzle” technique as possible approach for coordinate transform Need rebinning before final spectrum is extracted l1 l2 li

“Delta” correction for chromatic slit loss - depends on source shape and position within shutter must be user-controlled l1 l2 li l1 l2 li Collapse to 1-d spectrum - depends on source extent and background subtraction must be user-controlled For quick-look analysis, pipeline subtracts TBD “default” background

An outline of the NIRSpec pipeline RAW DATA BIAS & DARK SUBTRACTION PIXEL-TO-PIXEL DQE CORR. LOCATE EXTR. WINDOW THROUGHPUT CORRECTION (incl. L-flat, blaze function, transmission of optics, & “default” chromatic slit loss) MSA CONFIG. & DISTORTION MAP GW TELEMETRY GEOMETRIC DISTORTION (spatial) FINAL l CALIBRATION (dispersion solution) “DELTA” CHROMATIC SLIT LOSS CORR. ABSOLUTE FLUX CALIBRATION EXTRACT 1-D SPECTRUM SUBTRACT BACKGROUND FINAL 1D SPECTRUM FLATFIELD REF. CUBE GRATING EQUATION PHOTFLAM KEYWORD BIAS/DARK REF. FRAMES “P-FLAT” REF. FRAME LINEARITY CORR. LINEARITY “CALWEBB” (Assuming no l-dependence) (one window for every open shutter…. ) (output is re-sampled grid) “CALNIRSpecA” (one spectrum for every open shutter) “CALNIRSpecB” (defined by user or average) (defined by user) (erg/cm2/s/Å, sampled within variable slit aperture)